1. Field of the Invention
The invention relates to a semiconductor wafer having a front side coated by chemical vapor deposition (CVD) and to a method for producing this semiconductor wafer. The invention also relates to a device for supporting a semiconductor wafer during the deposition of a layer on a front side of the semiconductor wafer by chemical vapor deposition (CVD).
2. Background Art
During chemical vapor deposition, in particular the deposition of an epitaxial layer on a polished semiconductor wafer, two phenomena may occur, inter alia, which are known by the terms “autodoping” and “halo”.
In “autodoping”, dopants pass from the backside of the semiconductor wafer via the gas phase into the deposition gas, which is fed over the front side of the semiconductor wafer. They are then incorporated into the epitaxial layer, predominantly in the edge region of the front side of the semiconductor wafer, and therefore cause a more or less pronounced undesired radial variation in the resistivity of the epitaxial layer.
“Halo” refers to a scattered light effect which is caused by light-scattering structures on the backside of the semiconductor wafer and is observable by shining a collimated light beam onto the backside of the semiconductor wafer. The structures mark transitions, on the surface of the backside of the semiconductor wafer, at which regions with a native oxide layer adjoin regions where such an oxide layer is not present or is no longer present. These transitions occur when removal of the native oxide layer during the pretreatment in a hydrogen atmosphere (“pre-bake”) before the actual epitaxial deposition was incomplete. One possibility for quantifying this effect consists in a scattered-light measurement of the haze (turbidity, opacity), for example with an SP1 light scattering meter from KLA Tencor, in the so-called DNN (“DarkField Narrow Normal”) or DWN (“DarkField Wide Normal) channel.
In order to avoid problems with “autodoping”, U.S. Pat. No. 6,129,047 proposes to provide slits in the bottom of the susceptor's recess (“pocket”) holding the semiconductor wafer, the slits being arranged on the outer edge of the bottom of the susceptor. Dopants diffusing out from the backside of the semiconductor wafer can be removed from the reactor, without previously reaching the front side of the semiconductor wafer, by a flushing gas which is fed through slits in the susceptor onto the wafer backside.
According to U.S. Pat. No. 6,596,095 B2, small bores along the entire bottom surface of the susceptor serve the same purpose. Here again, the dopant diffusing out from the backside of the semiconductor wafer is transported away by feeding a flushing gas past the susceptor. These measures are also effective against “Halo” formation because they facilitate removal of the native oxide layer, since gaseous reaction products which are created by dissolving the native oxide are likewise transported away through the holes in the bottom and the flushing gas flowing past the susceptor.
DE 10328842 discloses a susceptor, which has a gas-permeable structure with a porosity of at least 15% and a density of from 0.5 to 1.5 g/cm3. By using such a porous susceptor, the gaseous reaction products which are formed during the pretreatment by dissolving the native oxide layer as well as the dopants diffusing from the semiconductor wafer to be coated, can escape through the pores of the susceptor to the backside of the susceptor, be taken up by a flushing gas flow, and thus be removed from the reactor. Using the described susceptor also avoids undesired nanotopography effects on the backside of the semiconductor wafer, which occur in the case of susceptors with holes. Holes in the susceptor affect the temperature field on the front side and backside of the semiconductor wafer to be coated, which leads to locally different deposition rates and finally to such nanotopography effects. The term nanotopography refers to height variations in the nanometer range, which are measured over a lateral extent of from 0.5 mm to 10 mm.
Another problem in the epitaxial coating of semiconductor wafers involves stresses in the epitaxially coated semiconductor wafers, which can lead to dislocations and slips. Several methods for identifying slips in semiconductor wafers are known: for example by visual inspection with collimated light, by means of devices for inspecting the surface of semiconductor wafers, or with devices which are suitable for determining the nanotopography.
The most sensitive method in this context, however, is SIRD (“Scanning Infrared Depolarization”) since not only slips but also photoelastic stresses can be measured by means of SIRD. The SIRD method for identifying stress fields, slips, sliplines, epitaxial defects, which is based on optical birefringence being introduced, is described for example in U.S. Pat. No. 6,825,487 B2.
Thermally induced stresses in epitaxially coated semiconductor wafers can be avoided during the epitaxial coating of semiconductor wafers by reducing the temperatures during the pretreatment steps in a hydrogen atmosphere (bake) and with the addition of hydrogen chloride to the hydrogen atmosphere (HCl etch) and in the actual coating step.
Lower coating temperatures, however, lead to an increased occurrence of undesired crystal defects such as stacking faults or typical epitaxial defects which are known by the terms “hillocks”, “mounds” or “pits”. At very low temperatures, polycrystalline growth may even take place. Another disadvantage is an inferior edge roll-off of the epitaxial layer as well as a deterioration in the local planarity of the semiconductor wafer (geometry, SFQR). The growth rate is furthermore reduced with lower deposition temperatures, which makes the process less economical. Reducing the pretreatment and deposition temperatures is therefore not acceptable owing to the associated disadvantages.
The prior art has not yet revealed any solution relating to the reduction of stresses, dislocations and slips in epitaxially coated semiconductor wafers with the high pretreatment and deposition temperatures which are categorically necessary, as explained above.